A method of the general class to which the invention relates is disclosed in German Patent Specification A-27 24 654 and Japanese Patent Specification A-26 410/1972.
In the method according to German Patent Specification the precalcination zone is divided, in the direction along which the material passes, into at least two reaction zones each of which is supplied separately with heat from the combustion of fuel. Before the material is delivered to the succeeding or downstream lower reaction zone it is separated from the hot gases of the upper reaction zone. Whereas only the exhaust gases from the combustion zone are delivered to the lower reaction zone, the upper reaction zone also receives combustion air from the cooling zone.
In the method according to the Japanese Patent Specification the two lowest cyclones of a multi-stage cyclone preheater form the precalcination zone which is supplied with additional fuel and through which the exhaust gases from the combustion zone (i.e., the rotary kiln) flow and to which additional combustion air, divided into the two branch air streams, is also delivered from the cooling zone.
In the known methods the exhaust gases from the combustion zone generally contain a more or less large proportion of nitrogen oxide (so-called NO.sub.x content). These nitrogen oxides are formed partly by reaction of oxygen with nitrogen in the combustion air and partly by oxidation of the nitrogen compounds in the fuel (cf. German periodical "Zement-Kalk-Gips", 6/1980, 310-313).
The NO.sub.x content of the exhaust gases from the combustion zone is undesirable, and not only because of the exhaust gas problems caused thereby. The object of the invention, therefore, is to improve the known methods so that the NO.sub.x content of the exhaust gases is reduced.